The preparation of thin epitaxial films on various substrates is an extremely important field in modern materials science and technology. Such films are applied for example in protective coatings, thin film devices, semiconducting devices, laser diodes, sensors, for data storage devices, and for mounting organic, including biological layers onto suitable substrates. The term "epitaxial" means the ordered growth of a material on the surface of a substrate or another layer, such that the crystalline properties and orientation of the deposited material reflects the orientation and crystalline structure of the substrate. Thus epitaxial deposition processes provide means to form thin specifically oriented crystalline layers.
A much addressed problem in epitaxial deposition is the one of lattice mismatch or misfit. Lattice mismatch occurs when the top and the bottom layer have different lattice constants. Deviations of less than 1 percent readily result in structural defects, including various types of dislocation, and the built-up strain in the deposited layer. These defects often affect the electronic properties of the grown layer in an often undesired manner. Accordingly, a number of solutions to this problem have been proposed in the art, including methods such as surface modification of the substrate by doping, ion implanting, or chemical reaction, deposition at elevated temperatures or of intermediate or buffer layers.
In the European Patent application EP-A-0 232 082, the substrate (Si) oriented in {100} direction is slightly tilted in the ffl001" direction to accommodate the lattice mismatch with GaAs. In addition to the tilting of the substrate, a buffer layer is provided to further reduce the misfit.
Another prior art approach utilizes the pseudomorphic growth of non-lattice-matched materials. In a system where there is sufficient attraction between the epilayer and the substrate and mismatch is low enough, initial epitaxial growth of a lattice mismatched material occurs two-dimensionally, with the epilayer conforming to the in-plane lattice structure of the substrate and with the mismatch accommodated by elastic strain. This growth is termed "commensurate" because it is growing with the lattice constant of the substrate, rather than with the unstrained bulk lattice constant of the epitaxial layer material. The commensurate growth is pseudomorphic as long as the layer thickness is below a critical thickness (which for a mismatch of 1-5% is about 1-100 nm) which defines the strain energy for which the introduction of dislocations becomes energetically favored.
The use of an intermediate layer is also disclosed in the U.S. Pat. No. 5,221,367. Here the growth of the intermediate layer is interrupted before the strain due to lattice mismatch is fully relieved by dislocations. The next layer is then grown in an unstrained and defect free condition where its lattice constant is about the same as the partially relieved lattice constant of the intermediate layer.
In yet another prior art approach, the desired lattice constant is obtained by growing several layers above their critical thickness, limiting however the change in lattice constant at each interface to about 1%. The grading from the lattice constant of the substrate material to the lattice constant of the final layer is in steps that are small enough to allow reliable two-dimensional growth throughout the growth process, This concept is for example pursued in U.S. Pat. No. 5,356,831, where a buffer layer is epitaxially grown on a ceramic substrate. The buffer layer has an elastically transitional lattice constant, matching at its lower surface the lattice constant of the ceramic substrate within a first given range, and matching at its upper surface the lattice constant of a following layer within a second given range. The buffer layer is provided by progressively growing a series of buffer layers of increasing lattice constant, each layer trapping its mismatch dislocations at its interface to the previous one, leaving its upper surface free of defects.
An effect known as "dislocation gettering" is applied in U.S. Pat. No. 5,294,808: If a heteroepitaxial layer is grown onto a sufficiently thin substrate, this effect causes any dislocations between the two layers to propagate into the substrate leaving the heteroepitaxial layer defect free.
To achieve an epitaxial growth of layers on a arbitrary, i.e. amorphous substrate, EP-A-0 352 931 teaches the use of a "seed" layer to be deposited onto the substrate before the deposition of a second (organic) crystalline layer.
However elaborated the above described solutions may be, in the art still the need is felt for an epitaxial structure accomodating a broad range of lattice constants and applicable to a variety of different materials.